FOR THE first time since 1974 there is talk that Congress may consider bills calling for major changes in campaign finance regulation. It isn't that the current legislation was defective from the start. It's that there is a hydraulic pressure behind money in campaigns, and when the inevitable loophole is found, the money comes gushing through. Not only the Democratic Study Group but conservative senators such as David Boren and Barry Goldwater are ready to tackle the subject again.

The sense that the system needs repair is strong in a DSG study showing an increase in PAC spending and a decrease in individual contributors. Legislators are uneasy lest it seem that organized interests are buying up Congress while the more diffuse interests of ordinary voters are not being as well served. PACs gave more than $100 million to congressional candidates in 1984, up from $12.5 million in 1972.

A new Boren-Goldwater bill would limit House candidates to $100,000 and Senate candidates to limits based on the size of their state. It would lower the maximum contribution for PACs and raise it for individuals. It would bar PACs from acting as conduits for individual contributions ("bundling") and tighten limits on spending by independent campaigns.

The DSG is preparing a bill that would provide a 100 percent tax credit on contributions up to $100. The current 50 percent credit on contributions up to $50 has just been deleted by the House Ways and Means Committee from its tax bill.

These proposals are in their formative stages. Congress, as it ponders them, should keep three things in mind. First, the key to any campaign finance law is full disclosure. Ensuring full disclosure may require tightening up the soft-money loophole and prohibiting practices such as bundling.

Second, reforms should not unduly restrict the amount of money that candidates, including challengers to incumbents, can raise. The system suffers not from a surfeit of campaigning but from a perception that the politically adept and economically interested have unfair advantages.

Third, reforms should not impinge on freedom of political expression. The argument of PAC organizers that PACs are only a vehicle for voters' self-expression is self-serving but cannot be casually dismissed. While Congress could further regulate that particular form of self-expression, it would be unwise to prohibit it altogether.

After such a long interval, Congress is wise to consider serious proposals for reform. But it should tread carefully, aware how difficult it is to anticipate all the consequences of change in the campaign finance law. The details matter.